1. Field of the Invention
The present invention relates to improvements in inertial navigation gyroscopes, and more particularly pertains to a new and improved two degree-of-freedom, free-rotor, gas-bearing gyroscope.
2. Description of the Prior Art
A typical free-rotor gyroscope is described in U.S. Pat. No. 3,025,708, "Free-Rotor Gyroscope Motor and Torquer Drives", issued Mar. 20, 1962, to J. M. Slater et al and assigned to the assignee of the present invention.
In such a gyroscope, the rotor is supported on a self-acting or autolubricated ball-and-socket gas bearing. A spherical support ball is typically the stationary part of the bearing while the socket, or concave element, is part of the rotor. When the gyro is subjected to acceleration, the gas bearing is deflected, i.e., the rotor is displaced from a centered position over the support ball. The resultant eccentricity of the bearing causes disturbance or error torques to be exerted on the rotor. The eccentricity of the bearing can be separated into a component parallel to the spin axis and components in the radial direction, i.e., normal to the spin axis.
Analysis has shown that the spin-axis component of bearing eccentricity is present as a factor in the significant terms of the expressions describing the acceleration-induced disturbance torques. The disturbance torques cause drift and errors. It is apparent then that the magnitudes of the disturbance torques are functions of the degree to which acceleration of the gyro displaces the rotor relative to the gas bearing along the spin axis.
In the prior art, forces directed axially to counteract the effects of acceleration and to center the rotor over the support ball have been provided by the bearing itself augmented by thrust pads formed in the spherical surface of the concave element on the rotor. However, the bearing and the thrust pads provide a relatively soft or compliant centering effect. They do not furnish that degree of stiffness to the gas bearing which would reduce the acceleration-induced errors to acceptable levels for some applications. In order to achieve the degree of navigational precision required for those applications, the outputs of the inertial sensors must be compensated. In the prior art, this has been accomplished using elaborate algorithms implemented electronically. The correction for gyro errors by compensation using, for example, electronic digital computers is relatively expensive and cumbersome. In addition, the requirement for the compensation unduly limits the applicability of the gas-bearing gyro. Therefore, it would be highly desirable to have the benefits of a free-rotor gas-bearing gyro in which acceleration-induced errors would be negligibly small. It has long been apparent that these benefits could not be supplied in such a gyroscope which relies only upon the gas-bearing and its thrust pads for maintaining the rotor axially centered over the support ball.
The gas bearing and thrust pads, of course, are only fully operative while the rotor is spinning at rated speed. The rotor lifts off radially on the bearing at relatively low spin speeds. However, somewhat higher spin speed is required for rotor levitation in the axial direction. Because of these factors, during gyro start-up and shut-down, the rotor tends to rub against the support ball. The resultant scuffing generates considerable wear debris which collects in the narrow bearing gap between the rotor and the support ball. Most of the wear debris is produced from scuffing between regions of the rotor and support ball near the shaft.
When the bearing deflects due to acceleration, the rotor tends to come in contact with the wear debris. This contact is an additional source of disturbance torques in the gyro. In addition, debris generation is one of the few failure modes of the gas-bearing free-rotor gyro. Hence, there has long been a desire to eliminate the generation of debris during starting and stopping in gas-bearing free-rotor gyros.
A typical eddy current motor drive for a gas-bearing free-rotor gyro includes a thin conductive cylindrical shell or sleeve on the rotor, the sleeve being centered on the spin axis. The sleeve is disposed within the gap of a polyphase stator for which the magnetic path is completed by a magnetic member spaced as close as possible to the conductive sleeve while preserving the small amount of tilt freedom necessary for the rotor. It is generally regarded as desirable to locate the motor and its sleeve relatively close to the spin axis at a selected diameter. At the selected diameter, disturbance effects due to direct hydrodynamic coupling and disturbance effects due to direct magnetic coupling tend to cancel each other.
A typical eddy current torquer drive for a gas-bearing free-rotor gyro includes a thin conductive cylindrical shell on the rotor disposed to operate in conjunction with a fixed set of electromagnets, differentially energized, opposed to a closely-spaced flux return member. The torquer construction thus resembles that of the motor drive. The motor sleeve and the torquer sleeve are typically mounted on opposite sides of the gyro rotor at the same selected diameter. This construction is regarded as desirable since it results in a structurally balanced configuration for the gyro rotor. With the exception of devices such as gyro pendulums in which the rotors are deliberately unbalanced, it is usually essential that the rotating member be precisely balanced radially, axially, and dynamically.
While the structurally balanced construction described above has the advantage of providing mechanical symmetry, it does not provide electromagnetic symmetry. A motor provides spin-axis torque at one side face of the rotor while a torquer provides torque about radial axes at the other side face of the rotor. Bias variations occur in the motor and the torquer when the gyro is subject to acceleration. They are due to elastic deflections of the housing and the support-ball shaft which distort the motor and torquer gaps. They are also due to the eccentric displacements of the motor and torquer sleeves occurring as a result of bearing deflection. These variations are error sources. It has long been desired to eliminate them.
Free-rotor gyros are subject to disturbances due to turbulence in the fluid medium surrounding the rotor. Means for reducing the random drift caused by these disturbances are disclosed in U.S. Pat. No. 3,706,231, "Turbulence and Vortex Suppression in Gyros", issued to Noar et al and assigned to Rockwell International Corporation the assignee of the present invention. Noar is a co-inventor herein. The reduction in fluid-medium vortex action and turbulence is accomplished by the introduction of certain structural protrusions and the like. However, this approach introduces added complexity and cost into the manufacture of the gyros. Therefore, there has long been a desire to reduce disturbances due to fluid-medium turbulence in a gas-bearing free-rotor gyro without the use of structural protrusions.